1. Field of the Invention
The present invention relates to a method for preparing magnetic particles for magnetic-recording media. More particularly, it relates to a method for preparing cobalt and ferrous iron surface modified .gamma.-ferric oxide particles.
2. Description of the Related Art
Acicular chromium dioxide and acicular cobalt-modified magnetic iron oxide are widely used as high density magnetic recording media, but acicular chromium dioxide has disadvantages in that a magnetic tape using this dioxide causes a higher degree wear of a magnetic head than a conventional magnetic tape using an iron oxide, and the recorded magnetization of medium deteriorate repeat by repeat of play back. Since the Curie point of chromium dioxide is very low (about 126.degree. C.), both the magnetization and coercivity decrease rapidly when the temperature is increased. Moreover, the problems of manufacturing cost and environment pollution, etc., arise. Acicular cobalt-containing magnetic iron oxide is generally available in two types; a so-called cobalt-doped magnetic iron oxide in which cobalt is diffused uniformly in the magnetic iron oxide particles, and a magnetic iron oxide in which cobalt is located only on the surface of the magnetic iron oxide particles. Of those two types of acicular cobalt-containing magnetic iron oxide particles, if cobalt content is same, the cobalt-doped type generally has a higher coercivity than cobalt surface modified type, viz., cobalt is located only on the surface of the particles. However, a magnetic tape is made, the cobalt-doped type acicular magnetic iron oxide particles have unstable magnetic problems and a problem of a level down of signal output by the times of play back, and have a high print-through level of the magnetic tape. The acicular magnetic iron oxide particles in which cobalt is coated only on the surface of the particles have improved these magnetic stabilities compared with the cobalt-doped type particles. The coercivity of the .gamma.-ferric oxide particles with cobalt coated only on the surface is almost linearly increased in proportion to the amount of the cobalt coated, and the print-through of a magnetic tape is improved by 2 to 8 dB when using the magnetic iron oxide particles having cobalt coated only on the surface of the particles, in comparison with the unmodified magnetic iron oxide particles. However, when the amount of cobalt is much increased, the magnetic properties of the particles are disadvantageously changed over a period of time, and the pressure and thermal stabilities are deteriorated. Further, reducing the amount of cobalt is desirable from the viewpoint of the manufacturing cost.
As the magnetic powder for recording media, materials having not only a higher coercivity and a higher saturation magnetization but also a lower electrical resistance are desired. Also, as tapes for a VHS type video tape recorder (VTR), the tapes must be black, and therefore, magnetic materials having a high blackness are desired. Namely, the magnetic materials must be as black as possible. In magnetic tapes, particularly VTR tapes, an electrostatic charge is caused by the friction associated with high speed running of the tape. The electrostatic charge disturb the smooth running of tape on recording or play back operation. Also the dropout is caused by dust adhesion. The S/N ratio is dropped by the noise of discharge. These problems caused by the generation of an electrostatic charge can be eliminated by making the magnetic tape electrically conductive.
To give the magnetic powder for recording media a higher saturation magnetization and a lower electrical resistance, ferrous ion should be added to the surface of the particles. But the addition of ferrous ion is disadvantageous in that it causes high print-through level of a magnetic tape.
As a method for improving the coercivity and the saturation magnetization of magnetic particles, a method has been proposed in which cobalt is coated on iron oxide particles, in the presence of barium or strontium salt (Japanese Unexamined Patent Publication (Kokai) No. 57-198607); and a method in which cobalt and ferrous iron are coated on iron oxide particles in the presence of barium or strontium salt (Japanese Unexamined Patent Publication (Kokai) No. 57-181102). These methods comprise the addition of an aqeous cobalt salt solution and aqueous barium or strontium salt solution into an aqueous dispersion of iron oxide particles, and further, in the latter method, an aqueous ferrous salt solution, followed by the addition of an alkaline and heating to cause chemical reaction and modification of the iron oxide particles with cobalt and the other metals.
Nevertheless, although the reaction rates in these methods are higher, for example, the reaction is completed in 3 to 4 hours at a reaction temperature of 100.degree. C., these methods have disadvantages in that the final product (magnetic particles) have a lower coercivity than the invention process product and the distribution of coercivity of the magnetic particles is wide. Because, it is difficult to uniformly coat cobalt on the surface of .gamma.-ferric oxide particles, which means that a large amount of cobalt must be coated on the particles to attain a required coercivity. The ununiformity of coating of cobalt results in a wider coercivity distribution of the modified .gamma.-ferric oxide particles.